Petrology of avachites, high-magnesian basalts of Avachinsky Volcano, Kamchatka: I. General characteristics and composition of rocks and minerals

• Main Authors: Portnyagin, Maxim, Plechov, P. Yu., Matveev, S. V., Osipenko, A. B., Mironov, N. L.
• Format: Article in Journal/Newspaper
• Language: English
• Published: MAIK 2005
• Subjects: Kamchatka
• Online Access: https://oceanrep.geomar.de/id/eprint/7110/; https://oceanrep.geomar.de/id/eprint/7110/1/Portnyagin-2005-Petrology_Abachites1__ENG.pdf; http://www.maik.rssi.ru/cgi-bin/search.pl?type=abstract&name=petreng&number=2&year=5&page=99

In 1935, A.N. Zavaritskii described high-magnesian [MgO/(MgO + FeO*) = 0.73–0.82] basalts and picrites, which are unique in Kamchatka and were found on Avachinsky volcano and eventually named ava- chites. This paper systematizes data on the composition of these rocks and presents the results of their detailed mineralogical examination on an electron microprobe (EMPA) and with the use of secondary-ion mass spec- trometry (SIMS) and vibrational IR spectrometry. The results thus obtained suggest that avachites are of volca- nic genesis and were produced by the crystallization of olivine (Fo91–80), clinopyroxene (Mg# = 92.5–73 mol %), and spinel [Mg# = 18–59 mol %, Cr/(Cr + Al) = 0.82–0.55] from a basaltic (SiO2 ≤ 52 wt %, MgO ~ 13 wt %) parental melt, whose composition was intermediate between those of island-arc ankaramites and high-Ca bon- inites. The high-Mg chemistry of the rocks (MgO = 14–20 wt %) is explained by the accumulation of olivine and pyroxene phenocrysts in an evolved basaltic melt (MgO ~ 5 wt %), which composes groundmass of ava- chites. The minerals crystallized under pressures of 1.0–0.1 GPa, at temperatures of ≤1380–1050°C, and an oxy- gen fugacity of ∆QFM = 0.5–2.0. The results of the IR spectroscopy of the olivine suggest that the parental magmas contained ≥0.5 wt % ç2é. The parental magmas of avachites were derived at high degrees (>20%) of the partial melting of a mantle source that was depleted more strongly than the source of mid-oceanic ridge basalts (MORB) and was metasomatized by a fluid or melt rich in LREE, Th, Ba, K, and Sr. The typical basaltic andes- ites of Avachinsky volcano can be genetically related to avachites and could be produced by olivine and pyrox- ene crystallization from parental melts of similar composition but under pressures varying within a narrower range. The composition of the associated primitive basalts indicates that the parental melts of the volcano were heterogeneous, and magmas of ankaramite composition could contribute to the genesis of volcanic series ...